1. Field of the Invention
This invention relates to a semiconductor device and a manufacturing process therefor.
2. Description of the Related Art
There have been recently intense efforts for investigating a semiconductor device comprising a capacitative element for preventing circuit oscillation or noise in a semiconductor LSI.
There are several methods for forming such a capacitative element structure in an LSI. Generally, a gate oxide film in a transistor (Tr) is often used as a dielectric for a capacitative element without its dedicated structure. As an example of such a technique, FIG. 2 shows a semiconductor device comprising a capacitative element having a conventional and common configuration.
In this semiconductor device 200, a capacitative element is formed using a gate oxide film 210 as a dielectric and a well 208 and a gate electrode 211 formed in a silicon substrate 207 as electrodes for the capacitative element. The capacitative element is separated from other elements by SiO2 regions 209a, 209b and 209c. The capacitative element and other elements are interconnected via contact plugs 212a, 212b and interconnections 206a, 206b. 
In this prior art, the semiconductor device 200 utilizes an existing common structure, so that increase of the number of manufacturing steps can be limited. Furthermore, a thin gate oxide film 210 is used as a dielectric, forming a capacitative element with a high capacity and a smaller area.
Alternatively, a structure dedicated to a capacitative element is sometimes used. As an exemple of such a technique, FIG. 3 shows a semiconductor device comprising a capacitative element having a conventional and common configuration.
In this semiconductor device 300, a capacitative element is formed by sequentially depositing a gate electrode 211 made of a metal material, a dielectric 213 (usually, a SiO2 or SiN film by CVD) and a an upper electrode for capacity 214 made of a metal material on an SiO2 region 209b formed on a silicon substrate 207. The capacitative element and other elements are interconnected via contact plugs 212a, 212b and interconnections 206a, 206b. 
In this prior art, the electrodes in the capacitative element are the gate electrode 211 and the upper electrode for capacity 214, both of which are made of a metal material, so that a capacitative element with less bias dependency can be formed.
Alternatively, a capacitative element can be formed using an interlayer insulating film as a dielectric and a metal interconnection as an electrode. As an exemple of such a technique, FIGS. 4A and 4B show a semiconductor device comprising a capacitative element having a conventional and common configuration.
A semiconductor device 400 shown in FIG. 4A is an example where a lower interconnection 201, an upper interconnection 204 and an interlayer insulating film 205 are used as capacitative elements. A semiconductor device 500 shown in FIG. 4B is an example using interconnections 206a, 206b and an interlayer insulating film 205 in the same layer as capacitative elements.
Both semiconductor devices 400, 500 shown in FIGS. 4A and 4B utilize an existing metal interconnection structure as it is, resulting in limitation in increase of the number of manufacturing steps in this prior art.
Additionally, Japanese Patent Application NO. 1999-274428 describes a technique for a semiconductor device with a capacitative element. FIG. 19 shows a semiconductor device similar to that in the reference. The semiconductor device comprises a lower electrode 413 on a silicon substrate 401. There is formed a silicon oxide film 402 between the silicon substrate 401 and the lower electrode 413. The lower electrode 413 has a structure where a titanium silicide film 412 is deposited on a polysilicon film 411.
On the lower electrode 413, there is formed an upper electrode 415. A dielectric film 406 is formed between the lower electrode 413 and the upper electrode 415, to provide a capacitative element. The capacitative element is surrounded by a first interlayer insulating film 404. The capacitative element and other elements are interconnected via aluminum interconnections 408a, 408b and a buried plug 409.
Japanese Patent Application NO. 1999-274428 describes that this prior art can provide a convenient process for manufacturing a semiconductor device to realize an improved yield and a reduced cost.
Japanese Patent Application NO. 2003-347416 discloses another technique for a semiconductor device comprising a capacitative element. FIGS. 18A and 18B show a semiconductor device similar to that described in the reference.
FIG. 18B is a cross-sectional view of this semiconductor device, which corresponds to a cross-sectional view taken on line A-A′ of FIG. 18A described below. The semiconductor device comprises a first interlayer insulating film, on which are formed lower interconnection metals 31, 37, on which an upper interconnection metal 3105 is deposited. There is formed a second interlayer insulating film 311 between the lower interconnection metals 31, 37 and the upper interconnection metal 3105.
The second interlayer insulating film 311 comprises a plurality of Ti-containing barrier metal/tungsten films 3104 connecting the lower interconnection metal 37 to the upper interconnection metal 3105. The bottoms and the sides of the Ti-containing barrier metal/tungsten films 3104 are surrounded by an insulating film for a capacitative element 3102. A capacitative element unit 3100 is constituted by the interconnection metal 37 and a Ti-containing barrier metal/tungsten film 3104 as electrodes and the insulating film for a capacitative element 3102 as a dielectric film.
The lower interconnection metal 31 and the upper interconnection metal 3105 are interconnected via an interconnection-connecting contact 3103.
FIG. 18A shows a side and a plan views of the upper interconnection metal 3105 in this semiconductor device. The bottom of the upper interconnection metal 3105 comprises a plurality of capacitative elements 3200 consisting of a Ti-containing barrier metal/tungsten film 3104 and the insulating film for a capacitative element 3102 to constitute the capacitative element unit 3100.
Japanese Patent Application NO. 2003-347416 describes that according to this prior art, a plurality of capacitative elements can be appropriately selected to adjust a capacity of a capacitative element.
The above prior art leaves room for improvement as described below.
First, the semiconductor device shown in FIG. 2 utilizes the well 208 formed in the silicon substrate 207 as an electrode. Thus, an applied voltage may generate a depletion layer in the well 208 and variation of a thickness of the depletion layer causes capacity variation, i.e., bias dependency. A designer must, therefore, design a circuit, taking such bias dependency into account.
Furthermore, the well 208 formed in the silicon substrate 207 and the gate electrode 211 are used as electrodes for a capacitative element, so that the capacitative element occupies a large area on a chip. Thus, a transistor (Tr) may not be formed in the area which is occupied by the capacitative element, leading to a larger chip area.
Second, the semiconductor device shown in FIG. 3 has a structure dedicated to a capacitative element. It may lead to increase of the number of manufacturing steps and thus an increased product cost in comparison with a device without such a dedicated structure.
In the structure shown in FIG. 4, the interlayer insulating film between metal interconnections used as a dielectric film is thicker than that in the other prior arts described above. It results in a capacity per a unit area of the dielectric film is lower and a larger area is, therefore, needed for giving a required capacity. Thus, it may lead to an increased chip size and an increased product cost.
Since a structure dedicated to a capacitative element is formed in the structure shown in FIG. 19, the number of manufacturing steps and a product cost may be increased in comparison with a device without such a dedicated structure.
In this configuration, the dielectric film 406 formed in the bottom of the upper electrode 415 can function as a component in a capacitative element, while the dielectric film 406 formed in the side of the upper electrode 415 cannot fully function as a component in a capacitative element. Thus, it may result in an inadequate capacity per a unit area of the capacitative element.
Since the structure shown in FIG. 18 also has a structure dedicated to a capacitative element, the number of manufacturing steps and a product cost may be increased in comparison with a device without such a dedicated structure.
The insulating film capacitative element 3102 formed in the bottom of the Ti-component barrier metal/tungsten film 3104 can function as a component in a capacitative element, while the insulating film for a capacitative element 3102 formed in the side of the Ti-component barrier metal/tungsten film 3104 cannot fully function as a component in a capacitative element. Thus, it may result in an inadequate capacity per a unit area of the capacitative element.